Field of the Invention
The present invention relates to an image capturing apparatus, such as a digital camera, and more particularly to image capturing control in flash photography in the image capturing apparatus configured to acquire phase difference information using an image sensor.
Description of the Related Art
In the flash photography in which an image is captured with flashlight emission that illuminates an object, it is necessary to precisely control a flashlight emission amount (or an irradiation light amount onto the object). In order to calculate a light emission amount in the main light emission in capturing an (or for exposure control), the pre-light emission (or pre-flash) is performed prior to the main light emission, and the light reflected from the object in the pre-light emission is often measured. Japanese Patent Laid-Open No. (“JP”) 2007-235893 discloses a precise exposure control method of the main light emission using the pre-light emission even in a scene with bright external light (so-called fill-in flash capturing).
The method disclosed in JP 2007-235893 uses an image sensor in which electric charge resetting and reading timings are different for each line, and an exposure period of an initial reading line and an exposure period of a final reading line partially overlap each other. A shift between the exposure timing of the initial reading line and the exposure timing of the final reading line is reduced by increasing the thinning number of reading lines.
An image sensor used for an image capturing apparatus can include a plurality of micro lenses, and a plurality of photoelectric conversion elements assigned to each micro lens. This type of image sensor can provide a plurality of (phase difference image) signals having a phase difference according to a focusing state of a capturing optical system, and the phase difference autofocus (“AF”) using the phase difference. In the flash photography using the image capturing apparatus having this image sensor, the phase difference AF information can be utilized to improve the precision of extracting the main object area to which the luminance is to be appropriately set. JP 2013-59082 discloses a method for acquiring a distance distribution for each area in the object image using a pair of phase difference image signals, and for reducing uneven flashlight distribution using this distance distribution information.
However, it takes a long time to read the signals separately out of the plurality of photoelectric conversion elements provided for each micro lens, in order to acquire a pair of phase difference image signals in the pre-light emission. As a result, it takes a long time to read the signal per one line. As a solution for this problem, as disclosed in JP 2007-235893, if the line thinning number is increased so that the exposure period of the initial reading line and the exposure period of the final reading line can partially overlap each other, the number of pixels in the image obtained by reading the signal is reduced and the photometric precision is lowered. As a result, an appropriate exposure control cannot be obtained.